Microresonator systems and methods of fabricating the same

ABSTRACT

Various embodiments of the present invention are related to microresonator systems that can be used as a laser, a modulator, and a photodetector and to methods for fabricating the microresonator systems. In one embodiment, a microresonator system comprises a substrate having a top surface layer, at least one waveguide embedded within the substrate, and a microdisk having a top layer, an intermediate layer, a bottom layer, current isolation region, and a peripheral annular region. The bottom layer of the microdisk is in electrical communication with the top surface layer of the substrate and is positioned so that at least a portion of the peripheral annular region is located above the at least one waveguide. The current isolation region is configured to occupy at least a portion of a central region of the microdisk and has a relatively lower refractive index and relatively larger bandgap than the peripheral annular region.

TECHNICAL FIELD

Embodiments of the present invention are directed to microresonatorsystems, and, in particular, to microresonator systems that can be usedas lasers, modulators, and photodetectors and to the methods forfabricating these systems.

BACKGROUND

In recent years, the increasing density of microelectronic devices onintegrated circuits has lead to a technological bottleneck in thedensity of metallic signal lines that can be used to interconnect thesedevices. In addition, the use of metallic signal lines yields asignificant increase in power consumption and difficulties withsynchronizing the longest links positioned on top of most circuits.Rather than transmitting information as electrical signals via signallines, the same information can be encoded in electromagnetic radiation(“ER”) and transmitted via waveguides, such as optical fibers, ridgewaveguides, and photonic crystal waveguides. Transmitting informationencoded in ER via waveguides has a number of advantages overtransmitting electrical signals via signal lines. First, degradation orloss is much less for ER transmitted via waveguides than for electricalsignals transmitted via signal lines. Second, waveguides can befabricated to support a much higher bandwidth than signal lines. Forexample, a single Cu or Al wire can only transmit a single electricalsignal, while a single optical fiber can be configured to transmit about100 or more differently encoded ER.

Recently, advances in materials science and semiconductor fabricationtechniques have made it possible to develop photonic devices that can beintegrated with electronic devices, such as CMOS circuits, to formphotonic integrated circuits (“PICs”). The term “photonic” refers todevices that can operate with either classically characterized ER orquantized ER with frequencies that span the electromagnetic spectrum.PICs are the photonic equivalent of electronic integrated circuits andmay be implemented on a wafer of semiconductor material. In order toeffectively implement PICs, passive and active photonic components areneeded. Waveguides and attenuators are examples of passive photoniccomponents that can typically be fabricated using conventional epitaxialand lithographic methods and may be used to direct the propagation of ERbetween microelectronic devices. Physicists and engineers haverecognized a need for active photonic components that can be used inPICs.

SUMMARY

Various embodiments of the present invention are related tomicroresonator systems comprising a microdisk that can be used as alaser, a modulator, and a photodetector and to methods for fabricatingthe microresonator systems. In one embodiment of the present invention,a microresonator system comprises a substrate having a top surfacelayer, at least one waveguide embedded within the substrate andpositioned adjacent to the top surface layer of the substrate, and amicrodisk having a top layer, an intermediate layer, a bottom layer,current isolation region, and a peripheral annular region. The bottomlayer of the microdisk is attached to and in electrical communicationwith the top surface layer of the substrate and is positioned so that atleast a portion of the peripheral annular region is located above the atleast one waveguide. The current isolation region is configured tooccupy at least a portion of a central region of the microdisk and has arelatively lower index of refraction and relatively larger bandgaps thanthe peripheral annular region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an isometric view of a first microresonator system inaccordance with embodiments of the present invention.

FIG. 1B shows a cross-sectional view of the first microresonator systemalong a line 1B-1B, shown in FIG. 1A, in accordance with embodiments ofthe present invention.

FIG. 2 shows a cross-sectional view of layers comprising an exemplarymicrodisk in accordance with embodiments of the present invention.

FIGS. 3A-3B show hypothetical plots of electronic bandgap energies forperipheral and current isolating regions of the microdisk shown in FIG.1.

FIG. 4A shows a path of current flow in a microdisk of the firstmicroresonator system, shown in FIG. 1, in accordance with embodimentsof the present invention.

FIG. 4B shows substantial confinement of a whispering gallery mode toperipheral regions of a microdisk of the first microresonator system,shown in FIG. 1, in accordance with embodiments of the presentinvention.

FIG. 5A shows an isometric view of a second microresonator system inaccordance with embodiments of the present invention.

FIG. 5B shows a cross-sectional view of the second microresonator systemalong line 5B-5B, shown in FIG. 5A, in accordance with embodiments ofthe present invention.

FIG. 6A shows paths of current flow in a microdisk of the secondmicroresonator system, shown in FIG. 5, in accordance with embodimentsof the present invention.

FIG. 6B shows substantial confinement of a whispering gallery mode toperipheral regions of a microdisk of the second microresonator system,shown in FIG. 5, in accordance with embodiments of the present invention

FIG. 7A shows an energy level diagram associated with quantizedelectronic energy states of a quantum well-based gain medium.

FIG. 7B shows a schematic representation of the first microresonatorsystem, shown in FIG. 1, operated as a laser in accordance withembodiments of the present invention.

FIG. 8A shows a schematic representation of the first microresonatorsystem, shown in FIG. 1, operated as a modulator in accordance withembodiments of the present invention.

FIG. 8B shows a plot of intensity versus time of unencodedelectromagnetic radiation.

FIG. 8C shows a plot of intensity versus time for a data encodedelectromagnetic radiation.

FIG. 9 shows a schematic representation of the first microresonatorsystem, shown in FIG. 1, operated as a photodetector in accordance withembodiments of the present invention.

FIGS. 10A-10K show isometric and cross-sectional views that areassociated with a method of fabricating the first microresonator system,shown in FIG. 1, in accordance with embodiments of the presentinvention.

FIGS. 11A-11B show cross-sectional views that are associated with amethod of fabricating the second microresonator system, shown in FIG. 5,in accordance with embodiments of the present invention.

DESCRIPTION OF EMBODIMENTS

Various embodiments of the present invention are related to microscaleresonator (“microresonator”) systems comprising a microdisk that can beused as a laser, a modulator, and a photodetector and to methods forfabricating the microresonator systems. In the various microresonatorsystem embodiments described below, a number of structurally similarcomponents comprising the same materials have been provided with thesame reference numerals and, in the interest of brevity, an explanationof their structure and function is not repeated.

FIG. 1A shows an isometric view of a microresonator system 100 inaccordance with embodiments of the present invention. Microresonatorsystem 100 comprises a microdisk 102 attached to a top surface layer 104of a substrate 106, a first electrode 108 attached to a top surface 110of microdisk 102, and a second electrode 112 attached to top surfacelayer 104 and positioned adjacent to microdisk 102. Microdisk 102 is amicroresonator of microresonator system 100 and can be configured tosupport certain WGMs. Substrate 106 includes two waveguides 114 and 116that extend through substrate 106 and are positioned adjacent to topsurface layer 104. Waveguides 114 and 116 are located beneath at least aportion of a peripheral annular region of microdisk 102. Microdisk 102comprises a top layer 118, a bottom layer 120, and an intermediate layer122 sandwiched between top layer 118 and bottom layer 120. Bottom layer120 can be comprised of the same material as top surface layer 104, asdescribed below with reference to FIG. 1B. Layers 118, 120, and 122 ofmicrodisk 102 are described in greater detail below with reference toFIG. 2.

FIG. 1B shows a cross-sectional view of microresonator system 100 alonga line 1B-1B, shown in FIG. 1A, in accordance with embodiments of thepresent invention. As shown in FIG. 1B, waveguides 114 and 116 arelocated beneath portions 124 and 126 of the peripheral annular region ofmicrodisk 102. Microdisk 102 includes a current isolation region 128configured to occupy at least a portion of a central region of microdisk102. Second electrode 112 is in electrical communication with bottomlayer 120 via top surface layer 104. Although only a single secondelectrode 112 is located on top surface layer 104 of substrate 106, inother embodiments of the present invention, two or more electrodes canbe positioned on top surface layer 104.

Note that the microresonators of the microresonator system embodimentsof the present invention are not limited to circular-shaped microdisks,such as microdisk 102. In other embodiments of the present invention,the microdisk 102 can be circular, elliptical, or have any other shapethat is suitable for supporting a WGM and creating resonant ER.

Top layer 118 can be a III-V semiconductor doped with an electronacceptor dopant, which is referred to as a “p-type semiconductor,” andbottom layer 120 can be a III-V semiconductor doped with an electrondonor dopant, which is referred to as an “n-type semiconductor,” whereRoman numerals III and V refer to elements in the third and fifthcolumns of the Periodic Table of the Elements. Intermediate layer 122includes one or more quantum wells. Each quantum well can be arelatively thin III-V semiconductor layer sandwiched between two layersof a different type of III-V semiconductor. FIG. 2 shows across-sectional view of layers comprising microdisk 102 in accordancewith embodiments of the present invention. In FIG. 2, top layer 118 canbe p-type InP, where Zn can be used as the dopant, and bottom layer 120can be n-type InP, where Si can be used as the dopant. Intermediatelayer 122 includes three quantum wells 201-203 ofIn_(x)Ga_(1-x)As_(y)P_(1-y), where x and y range between 0 and 1.Intermediate layer 122 also includes barrier layers 205-208 ofIn_(x)Ga_(1-x)As_(y)P_(1-y), where x and y range between 0 and 1. Thechoice of parameters x and y are made to lattice match adjacent layersand are well-known in the art. For example, for layers which arelatticed matched to InP layers 118 and 120, the x value is chosen to be0.47. The choice of y determines the bandgap energy of the quantum well.Operation of a quantum well is described below with reference to FIG.7A. The quantum wells 201-203 can be configured to emit ER at a desiredwavelength A while the barrier layers 205-208 can be configured to havea relatively larger bandgap in order to confine carriers (i.e.,electrons and holes) injected into the quantum well. Layers 205 and 206separate quantum wells 201-203, and layers 207 and 208 are tworelatively thicker layers that separate quantum wells 201 and 203 fromlayers 118 and 120, respectively. Substrate 106 can be comprised ofSiO₂, Si₃N₄ or another suitable dielectric insulating material.Waveguides 114 and 116 can be comprised of a column IV element, such asSi and Ge. In other embodiments of the present invention, other suitableIll-V semiconductor, such as GaAs, GaP or GaN, may be used.

Current isolation region 128 has a relatively larger electronic bandgapthan the quantum well electronic bandgaps associated with the peripheralannular region of microdisk 102. FIG. 3A shows a plot of electronicbandgap energies versus microdisk 102 height z for three quantum wellsof the peripheral regions of microdisk 102. The electronic bandgapenergies associated with bottom layer 120 and top layer 118 arerepresented by ΔE_(B) and Δ_(E) _(T), respectively. The quantum wells inintermedate layer 122 have bandgap energies ΔE_(QW), and the barrierlayers adjacent to the quantum well layers have larger bandgap energiesΔE_(Bar). Note that the bandgap energies ΔE_(T) and ΔE_(B) are largerthan the bandgap energy ΔE_(Bar), which corresponds to layers 118 and120 forming a double heterojunction barrier for confining electrons andholes to intermediate layer 122. FIG. 3B shows a plot of electronicbandgap energies versus microdisk 102 height z for current isolationregion 128 of microdisk 102. As shown in FIG. 3B, current isolationregion 128 eliminates or makes the bandgap energies associated with thequantum well layers and the barrier layers of intermediate layer 122indeterminate, as represented by dashed line energy levels 302 and 304.

The difference between the electronic bandgap energies associated withcurrent isolation region 128 and the peripheral region of microdisk 102can be used to substantially confine current to paths in the peripheralregion when a voltage is applied to electrodes 108 and 112. FIG. 4Ashows a path 402 representing the flow of current between electrodes 108and 112 in accordance with embodiments of the present invention. Path402 bends around higher bandgap current isolation region 128 inconnecting electrodes 108 and 112. Current can be substantially confinedto peripheral regions of microdisk 102, such as peripheral region 126,as follows. Consider applying a voltage to electrodes 108 and 112 thatis larger than the electronic bandgap energies associated with theperipheral annular region but does not exceed the electronic bandgapenergy associated with current isolation region 128. Because the voltageis large enough, current can flow through peripheral region 126, butcurrent cannot flow through current isolation region 128. In otherwords, current can be substantially confined to peripheral region 126using relatively lower voltages than the voltages needed for current toflow through current isolation region 128. Current paths avoidingcurrent isolation region 128, such as path 402, represent lower energypaths for current to flow along between electrodes 108 and 112.

In general, because a microresonator has a larger overall index ofrefraction than its surrounds, ER transmitted within microdisk typicallybecomes trapped as a result of total internal reflection near thecircumference of the microdisk. Modes of ER that are trapped near thecircumference of the microdisk are called “whispering gallery modes(‘WGMs’).” A WGM has a particular resonant wavelength λ that is relatedto the diameter of a microdisk. However, for a typical microdisk, othermodes exist which do not confine ER near the circumference in the formof a WGM.

Microdisk 102 embodiments of the present invention can be used tosubstantially confine ER to peripheral regions of microdisk 102 becausethe relatively wider bandgap, current isolation region 128 has arelatively lower index of refraction than the peripheral regions ofmicrodisk 102. FIG. 4B shows substantial confinement of a WGM toperipheral regions of microdisk 102 in accordance with embodiments ofthe present invention. As shown in FIG. 4B, a top view 404 of microdisk102 includes directional arrows located along the circumference ofmicrodisk 102. The directional arrows represent a propagation directionof a hypothetical WGM propagating near the circumference of microdisk102, and the length of the directional arrows corresponds to thewavelength λ of the WGM. Intensity plot 406 shows an intensitydistribution of a WGM versus distance along line A-A in top view 404.Dashed-line intensity curves 408 and 410 show the WGM substantiallyconfined near the circumference of microdisk 102. Portions of curves 408and 410 that extend beyond the diameter of microdisk 102 representevanescence of the WGM along the circumference of microdisk 102.Cross-sectional view 412 shows the portions 124 and 126 of theperipheral annular region occupied by the WGM. Dashed-line ellipses. 414and 416 show evanescent coupling of the WGM into waveguides 114 and 116.Thus current isolation region 128 provides for both current and opticalisolation since the ER will be confined in the region of higher index ofrefraction.

FIG. 5A shows an isometric view of a second microresonator system 500 inaccordance with embodiments of the present invention. Microresonatorsystem 500 is identical to microresonator system 100, shown in FIG. 1,except the first electrode 108 has been replaced by a microringelectrode 502 and a third electrode (not shown) is positioned on topsurface layer 104 of substrate 106 adjacent to microdisk 102. FIG. 5Bshows a cross-sectional view of microresonator system 500 along line5B-5B, shown in FIG. 5A, in accordance with embodiments of the presentinvention. As shown in FIG. 5B, microring electrode 502 is positionedabove and covers at least a portion of the top surface of microdisk 102.A third electrode 504 is positioned adjacent to microdisk 102 and is inelectrical communication with bottom layer 120 via top surface layer104.

Because microring electrode 502 is located above a portion of theperipheral region of microdisk 102, the flow of current betweenmicroring electrode 502 and electrodes 112 and 504 takes a more directpath than the path 402 shown in FIG. 4A. FIG. 6A shows paths 602 and 604representing the flow of current between microring electrode 502 andelectrodes 112 and 504 in accordance with embodiments of the presentinvention. Paths 602 and 604 represent more direct or lower resistanceroutes for current to flow between microring electrode 502 and secondand third electrodes 112 and 504 than path 402. As shown in FIG. 6B,substantial confinement of a WGM to the peripheral region of microdisk102 and evanescent coupling of the WGM into waveguides 114 and 116 isthe same as the description provided above with reference to FIG. 4B.

Microdisk 102 can be used as a laser that generates coherent ERtransmitted in waveguides 114 and 116. A laser comprises three basiccomponents: a gain medium or amplifier, a pump, and feedback of the ERinside an optical cavity. The quantum wells of intermediate layer 122comprise the gain medium, a current or voltage applied to electrodes 108and 112 is the pump, and feedback is created by total internalreflection as a WGM generated by pumping quantum wells of intermediatelayer 122 propagates near the circumference of microdisk 102.

A gain medium can be comprised of at least one quantum well with asuitable bandgap. The quantum well size and the bulk materialsurrounding the quantum well determine the energy level spacing ofelectronic states in the quantum well. Typically, the quantum well isconfigured to have a relatively small number of quantized electronicenergy levels in the valance band and a few quantized hole energy levelsin the conduction band. Electrons transitioning from the lowest energylevels in the conduction band to energy levels in the valance banddetermine the emission wavelength λ of the gain medium. FIG. 7A shows anenergy level diagram 700 associated with quantized electronic energystates of a quantum well-based gain medium of width α. Narrower region702 with bandgap energy E_(g) corresponds to a quantum well, andrelatively wider regions 704 and 706 with bandgap energy Ē_(g)correspond to bulk material surrounding the quantum well. As shown inFIG. 7A, the quantum well has a hole energy level 708 in the conductionband and three electronic energy levels 710-712 in the valence band.Because the gain medium comprises semiconductor material, an appropriateelectronic stimulus, such as electrical pumping, promotes electrons fromthe valance band into the quantized levels in the conduction band, suchas hole energy level 708. Spontaneous recombination of an electron inthe conduction with a hole in the valance band results in emission of aphoton having an energy given by hc/λ, where h is Plank's constant, andc is the speed of ER in a vacuum. A stimulated emission occurs as aresult of photons in the WGM stimulating the gain medium to generatemore photons at the same energy or wavelength. In both spontaneous andstimulated radiative emissions, the energy of the ER emitted is:

${E_{2} - E_{1}} = \frac{hc}{\lambda}$

where E₂ is the energy level 708 of the electrons that have been pumpedinto the conduction band, and E1 is the energy level 710 associated withholes in the valance band that combine with electron from the conductionband. As long as the electrical pump is applied to the gain medium,feedback caused by total internal reflection within microdisk 102 causesthe intensity of the WGM to increase. Lasing occurs when the gain equalsthe loss inside microdisk 102. Microdisk 102 forms the optical cavitywith gain, and the waveguides 114 and 116 couple the ER out of microdisk102.

FIG. 7B shows a schematic representation of the first microresonatorsystem, shown in FIG. 1, operated as a laser in accordance withembodiments of the present invention. As shown in FIG. 7B, electrodes108 and 112 are connected to a current source 710. Quantum-well layersof microdisk 102 can be operated as a gain medium by pumping microdisk102, as described above with reference to FIG. 7A, with a current of anappropriate magnitude supplied by current source 710. As a result, a WGMhaving a wavelength λ is generated within microdisk 102, and totalinternal reflection causes the WGM to propagate near the circumferenceof microdisk 102 as the intensity of the WGM increases. The WGMevanescently couples into the waveguides 114 and 116 yielding ER with awavelength λ that propagates in waveguides 114 and 116.

FIG. 8A shows a schematic representation of the microresonator system100, shown in FIG. 1, operated as a modulator in accordance withembodiments of the present invention. Current source 710 is connected toa data source 802, which can be a central processing unit, memory, oranother data generating device. ER source 804 is coupled to waveguide116 and emits ER with a substantially constant intensity over time, asshown in FIG. 8B. Returning to FIG. 8A, the amount of ER coupled intomicrodisk 102 depends on the detuning, the coupling coefficient, and thelosses inside of microdisk 102. When the wavelength λ of the ER emittedby source 804 is detuned from the resonance of microdisk 102, the ERdoes not couple from waveguide 116 into microdisk 102. When thewavelength λ of the ER is at resonance with microdisk 102, thetransmission of the ER propagating in the waveguide 116 is reducedbecause the ER is evanescently coupled into microdisk 102 creating aWGM. A portion of the ER transmitted in waveguide 116 evanescentlycouples into the peripheral region of microdisk 102 located abovewaveguide 116 and propagates in the peripheral region as a WGM with awavelength λ. Data source 802 encodes data in the WGM by modulating themagnitude of the current generated by current source 710. Modulating themagnitude of the current transmitted between electrodes 108 and 112causes the index of refraction of microdisk 102 to correspondinglychange. When the index of refraction of microdisk 102 is changed, theresonant wavelength of microdisk 102 changes causing a detuning from theresonant wavelength of ER transmitted in waveguide 116. This in turnmodulates the transmission of ER from waveguide 116 into microdisk 102and subsequently modulates the intensity of the ER transmitted inwaveguide 116. When waveguide 114 is present, ER can be transferred towaveguide 114 from the input waveguide 116 via microdisk 102. The amountof ER transferred to waveguide 114 depends on the coupling strength.Modulating the index of refraction of microdisk 102 results in areduction in the intensity of the ER transmitted to waveguide 114. Onecan also modulate the intensity of the ER in waveguide 116 by adjustingthe loss inside microring 102. This is accomplished by using the quantumconfined stark effect which modulates the bandgap of the quantum wellthrough the application of an applied voltage. Increasing the loss inmicrodisk 102 modulates the intensity transmitted past microdisk 102 inwaveguides 114 and 116.

FIG. 8C shows intensity versus time of modulated ER where relativelylower intensity regions 806 and 808 correspond to a relatively higherindex of refraction induced on microdisk 102. The relative intensitiescan be used to encode information by assigning a binary number to therelative intensities. For example, the binary number “0” can berepresented in a photonic signal by low intensities, such as intensityregions 806 and 808, and the binary number “1” can be represented in thesame photonic signal by relatively higher intensities, such as intensityregions 810 and 812.

FIG. 9 shows a schematic representation of the first microresonatorsystem, shown in FIG. 1, operated as a photodetector in accordance withembodiments of the present invention. In this configuration, the bandgapof the quantum wells is chosen to be less than the source of radiationof the input ER transmitted in waveguide 116. A reverse bias can also beapplied to the electrodes so that an electric field is present insidethe microresonator. The incoming ER coupled to the microresonator willbe absorbed inside the quantum well generating an electron hole pair.The electric field inside the microring is such that the electrons andholes are separated and a current is generated at the electrodes 108 and112. Modulated ER λ encoding information is transmitted in waveguide116. The ER evanescently couples into the peripheral region of microdisk102 producing a corresponding modulated WGM. Fluctuations in theintensity of the WGM propagating in the peripheral region induces acorresponding fluctuating current between electrodes 108 and 112. Thefluctuating current is an electrical signal encoding the same dataencoded in the modulated ER, which is processed by computational device902.

FIGS. 10A-10K show isometric and cross-sectional views that areassociated with a method of fabricating microresonator system 100, shownin FIG. 1, in accordance with embodiments of the present invention. FIG.10A shows an isometric view of a first structure 1000 comprising a toplayer 1002, an intermediate layer 1004, a bottom layer 1006, and an etchstop layer 1008 supported by a phosphorus-based wafer 1010. Layers 1002and 1006 can be comprised of n-type and p-type III-V semiconductors,such as InP or GaP doped with Si and Zn, respectively. Intermediatelayer 1004 includes at least one quantum well, as described above withreference to FIG. 2. Etch stop layer 1008 can be a thin layer oflatticed matched In_(0.53)Ga_(0.47)As. Layers 1002, 1004, and 1006 canbe deposited using molecular beam expitaxy (“MBE”), liquid phase epitaxy(“LPE”), hydride vapor phase epitaxy (“HVPE”), metalorganic vapor phaseexpitaxy (“MOVPE”), or another suitable expitaxy method. FIG. 10B showsa cross-sectional view of layers 1002, 1004, 1006, 1008, and wafer 1010.

Next, as shown in the cross-sectional view of FIG. 10C, sputtering canbe used to deposit an oxide layer 1012 over top layer 1002. Oxide layer1012 can be used to facilitate wafer bonding of top layer 1002 ontosubstrate 106, as described below with reference to FIG. 10G. Layer 1012can be SiO₂, Si₃N₄, or another suitable dielectric material thatsubstantially enhances wafer bonding to substrate 106.

FIG. 10D shows a silicon-on-insulator substrate (“SOI”) wafer 1014having a Si layer 1016 on an oxide substrate layer 1018. Siliconwaveguides 114 and 116 can be fabricated in Si layer 1016 as follows. Aphotoresist can be deposited over Si layer 1016 and a photoresist maskof waveguides 114 and 116 can be patterned in the photoresist using UVlithography. Waveguides 114 and 116 can then be formed in Si layer 1014using a suitable etch system, such as inductively coupled plasma etcher(“ICP”), and a low-pressure, high-density etch system with a chemistrybased on Cl₂/HBr/He/O₂. After waveguides 114 and 116 have been formed inSi layer 1016, a solvent can be used to remove the photoresist maskleaving waveguides 114 and 116, as shown in FIG. 10E. An oxide layercomprised of the same oxide material as substrate 1018 can be depositedover waveguides 114 and 116 using liquid-phase, chemical-vapordeposition. Chemical mechanical polishing (“CMP”) processes may be usedto planarize the deposited oxide in order to form substrate 106 withembedded waveguides 114 and 116, as shown in the cross-sectional view ofsubstrate 106 in FIG. 10F.

Next, as shown in FIG. 10G, first structure 1000 is inverted and waferbonding is used to attach oxide layer 1012 to the top surface ofsubstrate 106. Selective wet etching can be used to remove layer 1010 inorder to obtain a second structure 1020 shown in FIG. 10H. Etch stoplayer 1008 can be included to stop the etching process from reachinglayer 1006. Hydrochloric acid can also be used to remove an InP-basedwafer 1010 because there is an etch selectivity between the InP and theInGaAs of etch stop layer 1008.

Next, reactive ion etching (“RIE”), chemically assisted ion beam etching(“CAIBE”), or inductively coupled plasma (“ICP”) etching can be used toetch layers 1002, 1004, and 1006 into the form of microdisk 102, asshown in FIG. 101. A portion of layer 1002 adjacent to substrate 106 isleft in order to from top surface layer 104.

FIG. 10J shows a cross-sectional view of microdisk 102 and substrate 106along a line 10J-10J, shown in FIG. 101. Current isolation region 128 isformed in at least a portion of a central region of the microdisk 102using impurity induced disording (“IID”) and annealing. IID methods arewell-known in the art and are described in “A quantum-well-intermixingprocess for wavelength-agile photonic integrated circuits,” by E. J.Skogen et al., IEEE J of Selected Topics in Quantum Electronics, Vol. 8,No. 4, 2002. IID intermixes the different compositions of the layers118, 120, and 122 where the impurity is introduced. After annealing, theintermixed region's bandgap shifts to a relatively larger bandgap.Impurities can be introduced in the desired regions by masking andemploying standard photolithographic processes.

After IID, a dopant can be implanted in top layer 118 to form a p-typesemiconductor top layer 118 since the IID will also tend to reduce thedoping level of a disordered region. For example, Zn serves as a p-typesemiconductor dopant for top layer 118 comprised of InP. As shown inFIG. 10K, material comprising first electrode 108 and second electrode112 can be deposited by e-beam evaporation, and patterned to formelectrodes 108 and 112 using standard photolithographic processes. Ametal with a p-type dopant, such as AuZn, can be used in first electrode108 to obtain a p-type contact, and a metal with an n-type dopant, suchas AuGe, can be used in second electrode 112 to obtain an n-type contact112.

FIGS. 11A-11B show cross-sectional views that are associated with amethod of fabricating photonic system 500, shown in FIG. 5, inaccordance with embodiments of the present invention. Forming microdisk102 and forming waveguides 114 and 116 in substrate 106 can beaccomplished as described above with reference to FIGS. 10A-10I. Asshown in FIG. 11A, material comprising microring electrode 502 andelectrodes 112 and 504 can be deposited and patterned as described abovewith reference to FIG. 10K. A metal with a p-type dopant can be used infirst electrode 108 to obtain a p-type contact, and a metal with ann-type dopant can be used in electrodes 112 and 504 to obtain n-typecontacts. Next, a mask layer can be placed over microring electrode 502and IID can be used to form current isolation region 128 in microdisk102. After IID, a dopant can be implanted in top layer 118 to formp-type top layer 118, as described above with reference to FIG. 10J

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that the specificdetails are not required in order to practice the invention. Theforegoing descriptions of specific embodiments of the present inventionare presented for purposes of illustration and description. They are notintended to be exhaustive of or to limit the invention to the preciseforms disclosed. Obviously, many modifications and variations arepossible in view of the above teachings. The embodiments are shown anddescribed in order to best explain the principles of the invention andits practical applications, to thereby enable others skilled in the artto best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the followingclaims and their equivalents:

1. A microresonator system comprising: a substrate having a top surface layer; at least one waveguide embedded within the substrate and positioned adjacent to the top surface layer of the substrate; and a microdisk having a top layer, an intermediate layer, a bottom layer, current isolation region and a peripheral annular region, wherein the bottom layer of the microdisk is attached to and in electrical communication with the top surface layer of the substrate, the microdisk is positioned so that at least a portion of the peripheral annular region is located above the at least one waveguide, and the current isolation region configured to occupy at least a portion of a central region of the microdisk and having a relatively lower index of refraction and relatively larger bandgap than the peripheral annular region.
 2. The system of claim 1 further comprising: a first electrode located on the top surface layer of the microdisk; and at least one second electrode located on the top surface layer of the substrate adjacent to the microdisk.
 3. The system of claim 2 wherein the first electrode further comprises a microring electrode configured to cover at least a portion of the peripheral region of the top surface of the microdisk.
 4. The system of claim 1 wherein the microdisk further comprises: a top layer; a bottom layer; and an intermediate quantum-well layer sandwiched between the top semiconductor layer and the bottom semiconductor layer.
 5. The system of claim 4 wherein the top layer further comprises a p-type semiconductor and the bottom layer further comprise an n-type semiconductor.
 6. The system of claim 4 wherein the intermediate layer further comprises at least one quantum well.
 7. The system of claim 1 wherein the microdisk further comprises one of: a circular shape; an elliptical shape; and any other shape that is suitable for supporting a whispering gallery mode.
 8. A laser comprising the microresonator system configured in accordance with claim
 1. 9. A modulator comprising the microresonator system configured in accordance with claim
 1. 10. A photodetector comprising the microresonator system configured in accordance with claim
 1. 11. A microdisk comprising: a top layer; a bottom layer; an intermediate layer having at least one quantum well, the intermediate layer sandwiched between the top layer and the bottom layer; a peripheral annular region including at least a portion of the top, intermediate, and bottom layers; and a current isolation region configured to occupy at least a portion of a central region of the microdisk including at least a portion of the top, intermediate, and bottom layers and having relatively lower index of refraction than the peripheral annular region.
 12. The system of claim 11 wherein the top layer further comprises a p-type semiconductor and the bottom layer further comprise an n-type semiconductor.
 13. The system of claim 11 wherein the microdisk further comprises one of: a circular shape; an elliptical shape; and any other shape that is suitable for supporting a whispering gallery mode.
 14. A method of fabricating a microresonator system, the method comprising: forming a multilayer system having a bottom layer, a top layer, and an intermediate layer having one or more quantum wells and sandwiched between the bottom layer and the top layer; embedding at least one waveguide in a substrate having a top surface, the at least one waveguide positioned adjacent to the top surface of the substrate; wafer bonding the top layer of the multilayer system to the top surface of the substrate; forming a microresonator in the multilayer system, wherein at least a portion of a peripheral annular region of the microresonator is portioned above the at least one waveguide; and forming a current isolation region in at least a portion of a central region of the microresonator.
 15. The method of claim 14 wherein forming the multilayer system further comprises: depositing a p-type semiconductor bottom layer on a second substrate; depositing one or more quantum-well layers on the bottom layer; and depositing an n-type semiconductor top layer on the one or more quantum-well layers.
 16. The method of claim 14 wherein depositing the top, the bottom, and the one or more quantum-well layers further comprises employing one of: molecular beam expitaxy; liquid phase expitaxy; hydride vapor phase expitaxy; metalorganic vapor phase expitaxy; and another suitable expitaxy method.
 17. The method of claim 14 wherein embedding waveguides in the substrate further comprises: providing a silicon-on-insulator substrate having a silicon layer and an oxide layer; depositing a photoresist layer on the silicon layer; forming a photoresist mask of waveguides in the photoresist layer; etching waveguides in the silicon layer; removing the photoresist mask; and depositing an oxide layer over the waveguides.
 18. The method of claim 14 wherein wafer bonding the top layer of the multilayer system to the top surface of the substrate further comprises depositing a first oxide layer on a top layer of the multilayer system and depositing a second oxide layer on the top surface of the substrate.
 19. The method of claim 14 wherein forming the microresonator in the multilayer system further comprises employing one of: reactive ion etching; chemically assisted ion beam etching; and inductively coupled plasma.
 20. The method of claim 14 wherein forming the current isolation region in at least a portion of the central region of the microresonator further comprises impurity induced disording and annealing. 